The host cell surface has a diverse repertoire of immune molecules to detect and attack foreign particles, including those introduced upon viral infections. Viruses, in turn, have developed an equally impressive arsenal of methods to evade host defenses, such as hijacking cellular membrane trafficking machinery to downregulate host innate and adaptive immune molecules. For example, HIV removes its primary receptor, CD4, from cellular membranes to avoid interference with viral release and infectivity; it prevents MHC-I from reaching the cell surface to evade immune surveillance by cytotoxic T cells; and it removes the innate host restriction factor BST2 (also known as tetherin) from the cell surface to allow for the efficient release of progeny virions. HIV Vpu, a transmembrane protein, and HIV Nef, a peripheral membrane protein, accomplish these tasks by linking targeted proteins to components of the host protein trafficking machinery, thereby inactivating host defense proteins through mislocalization and degradation. The focus of this proposal is to establish the mechanisms by which Nef and Vpu hijack host membrane trafficking pathways to down regulate the expression of immune molecules at the cell surface. Our approach includes structure determination by X-ray crystallography and single particle electron microscopy. It features novel fusion-protein strategies that allow the investigation of membrane-mediated interactions in aqueous solution. It features cell biologic and virologic validation of the complexes at both the molecular and structural levels. Our work will significantly advance our understanding of a diverse range of host-viral interactions at cellular membranes and identify new antiviral drug-targets. Inhibition of these targets would disable viral modulation of cellular membranes and potentially empower host immunity to more effectively control an HIV infection. Moreover, the experimental systems devised for our research project will provide valuable new tools for the studies of host-pathogen relationships and membrane protein interactions.